Printed circuit board and switching power supply employing it

ABSTRACT

A printed circuit board has: a first wiring pattern laid in a first layer such that, when a predetermined component is mounted in a predetermined mounting region, a first current path in an open ring shape leading from a first end to a second end is formed; a second wiring pattern laid in a second layer different from the first layer such that a second current path in an open ring shape leading from a third end to a fourth end is formed; a first conductive member formed between the second and third ends; and a second conductive member formed between the first and fourth ends. The first and second wiring patterns are so laid that, as seen in their respective plan views, the directions of the currents flowing across the first and second current paths, respectively, are opposite to each other.

This application is a continuation of U.S. application Ser. No.15/912,907, filed Mar. 6, 2018, which is based on Japanese PatentApplication No. 2017-042891 filed on Mar. 7, 2017, the contents of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention disclosed herein relates to printed circuit boards, and toswitching power supplies employing printed circuit boards.

2. Description of Related Art

A switching power supply includes a path in which an abrupt change incurrent occurs as a result of the on/off control of a switching outputstage. The path generally has a capacitance component and an inductancecomponent; thus, ringing occurs due to LC resonance, and this causes EMI(electromagnetic interference) radiation noise (switching noise) to beemitted to the outside.

FIG. 16 is a diagram showing a first conventional example of a switchingpower supply. The switching power supply of this conventional examplehas, on a printed circuit board 200, a bypass capacitor C10 disposednear a semiconductor device U10 (controller IC).

To be sure, with this circuit board layout, it is possible to reduce theparasitic capacitance component Lx that accompanies the wiring patternand to reduce the path in which an abrupt change in current occurs (thatis, the input loop that leads from the first terminal of the bypasscapacitor C10 via high-side and low-side switches QH and QL inside thesemiconductor device U10 back to the second terminal of the bypasscapacitor C10), and thus it is possible to weaken the magnetic fieldthat occurs in the input loop (in the illustrated example, the magneticfield that points from the near side to the far side of the plane of thediagram). Inconveniently, with this approach against EMI, there is alimit to shortening the distance between the bypass capacitor C10 andthe semiconductor device U10, and it is not possible to completelyeliminate the magnetic field that occurs in the input loop.

FIG. 17 is a diagram showing a second conventional example of aswitching power supply. The switching power supply of this conventionalexample is proposed in Japanese Patent registered as No. 5779213, andhas, inside a semiconductor device U20, high-side and low-side switchesQH and QL each split into two parts, namely high-side switches QH1 andQH2 and low-side switches QL1 and QL2 respectively so that a pair ofinput loops around which currents flow in mutually opposite directionsis provided symmetrically left to right in the same layer. In thisconfiguration, the magnetic fields that occur in those input loopsrespectively cancel (neutralize) each other.

To be sure, with this conventional technology, it is possible to cancelthe magnetic fields in the direction perpendicular to the plane of thediagram, and it is thus possible to reduce the EMI radiation noiseemitted outside the switching power supply. Inconveniently, with thisapproach against EMI, two bypass capacitors C21 and C22 are required,and this leads to an unnecessary increase in cost. Also inconveniently,with the above-mentioned pair of input loops, the magnetic fields tocancel each other are disposed away from each other across thesemiconductor device U20, and thus those magnetic fields are noteliminated completely.

On the other hand, according to one conventionally known approachagainst EMI, spectrum spreading is applied to the driving frequency of aswitching power supply. Inconveniently, spectrum spreading has only alimited effect in suppressing EMI radiation noise, and does not providea thorough approach against EMI.

SUMMARY OF THE INVENTION

In view of the problems encountered by the present inventors, an objectof the invention disclosed herein is to provide a printed circuit boardthat can reduce EMI radiation noise from a switching power supply, andto provide a switching power supply employing such a printed circuitboard.

According to one aspect of what is disclosed herein, a printed circuitboard includes: a first wiring pattern which is laid in a first layersuch that, when a predetermined component is mounted in a predeterminedmounting region, a first current path in the shape of an open ring thatleads from a first end to a second end is formed; a second wiringpattern which is laid in a second layer, which is different from thefirst layer, such that a second current path in the shape of an openring that leads from a third end to a fourth end is formed; a firstconductive member which is formed between the second end and the thirdend; and a second conductive member which is formed between the firstend and the fourth end. Here, the first and second wiring patterns areso laid that, as seen in their respective plan views, the direction ofthe current that flows across the first current path and the directionof the current that flows across the second current path are opposite toeach other.

Other features, elements, steps, benefits, and characteristics of thepresent invention will become clearer with reference to the followingdescription of preferred embodiments thereof in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a principal part of aswitching power supply (with a high-side switch on);

FIG. 2 is a diagram showing a configuration of a principal part of aswitching power supply (with a high-side switch off);

FIG. 3 is a diagram showing a configuration of a principal part of aswitching power supply (a differential current);

FIG. 4 is a diagram showing voltages and currents at relevant points ina switching power supply;

FIG. 5 is a diagram illustrating the principle of how EMI radiationnoise occurs;

FIG. 6 is a diagram showing a planar layout of a first wiring pattern;

FIG. 7 is a diagram showing a planar layout of a second wiring pattern;

FIG. 8 is a diagram showing a spatial layout of a 3D wiring pattern;

FIG. 9 is a diagram showing the direction of a current flowing across a3D wiring pattern;

FIG. 10 is a diagram showing a magnetic field canceling effect (with notwist) at side AE;

FIG. 11 is a diagram showing a magnetic field canceling effect (with atwist) at side AE;

FIG. 12 is a diagram showing a magnetic field canceling effect betweenfaces ABCD and EFGH;

FIG. 13 is a diagram showing a magnetic field canceling effect betweenfaces ADFE and BCGH;

FIG. 14 is a diagram showing a magnetic field canceling effect betweenfaces ABHE and DCGF;

FIG. 15 is a diagram showing a 3D wiring pattern with power MOSFETs eachsplit into two parts;

FIG. 16 is a diagram showing a first conventional example of a switchingpower supply; and

FIG. 17 is a diagram showing a second conventional example of aswitching power supply.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Switching Power Supply: FIGS. 1 to 3 are diagrams each showing aconfiguration of a principal part of a switching power supply. FIG. 1shows a path across which a high-side switching current IHG flows, andFIG. 2 shows a path across which a low-side switching current ILG flows.FIG. 3 shows a differential current (=IHG−ILG) between the high-side andlow-side switching currents IHG and ILG.

As shown in FIGS. 1 to 3, the switching power supply 1 of thisconfiguration example is a step-down DC/DC converter that includes ahigh-side switch QH and a low-side switch QL (in the illustratedexample, both are N-channel power MOSFETs (metal-oxide-semiconductorfield-effect transistors)), an input capacitor Ci, an output capacitorCo, a bypass capacitor Cb, and an output coil Lo, and that generatesfrom an input voltage Vi a desired output voltage Vo to supply it to aload. The load is anything that is incorporated along with the switchingpower supply 1 in various kinds of electronic appliances, and operatesby being fed with direct-current power (the output voltage Vo and anoutput current Io) from the switching power supply 1.

In the switching power supply 1 of this configuration example, thehigh-side and low-side switches QH and QL, which constitute a switchingoutput stage, are controlled to turn on and off complementarily with ahigh-side gate signal HG and a low-side gate signal LG respectively. Inthe present specification, “complementarily” covers not only operationwhere the on/off states of the high-side and low-side switches QH and QLare completely reversed but also operation where periods (dead time) inwhich the high-side and low-side switches QH and QL are simultaneouslyoff are reserved.

Through the on/off control described above, a switching voltage SW witha square waveform that is pulse-driven between the input voltage Vi anda ground voltage GND appears at the connection node between thehigh-side and low-side switches QH and QL. The output coil Lo and theoutput capacitor Co function as an LC filter that rectifies and smoothsthe switching voltage SW to generate the output voltage Vo.

FIG. 4 is a diagram showing voltages and currents at relevant points inthe switching power supply 1, depicting, from top down, the inputvoltage Vi, the switching voltage SW, the high-side switching currentIHG, the low-side switching current ILG, a coil current IL, and theoutput voltage Vo.

As illustrated, in the high-level period of the switching voltage SW(with QH on and QL off), the high-side switching current IHG (and hencethe coil current IL and the output current Io) flows across a currentpath (see FIG. 1) via the high-side switch QH. On the other hand, in thelow-level period of the switching voltage SW (with QH off and QL on),the low-side switching current ILG (and hence the coil current IL andthe output current Io) flows across the current path (see FIG. 2) viathe low-side switch QL.

Thus, the switching power supply 1 includes a path in which an abruptchange in current occurs as a result of the on/off control of thehigh-side and low-side switches QH and QL. In particular, the input loopvia the bypass capacitor Cb (the hatched region in FIG. 3) acts as asource of EMI radiation noise.

FIG. 5 is a diagram illustrating the principle of how EMI radiationnoise occurs. As illustrated, the wiring of the input loop (see thehatched region in the diagram) via the bypass capacitor Cb isaccompanied by a parasitic inductance component Lx of about 1 nH permillimeter. Moreover, the drain-source channel of each of the high-sideand low-side switches QH and QL is accompanied by a parasiticcapacitance component Cx. On the other hand, the on/off transitionperiod of each of the high-side and low-side switches QH and QL (andhence the rise time and the fall time of the switching voltage SW) isabout several nanoseconds (V=L×dI/dt, I=C×dV/dt).

Accordingly, as indicated in a balloon in the diagram, when theswitching voltage SW rises and falls, severe ringing occurs due to LCresonance (at a resonance frequency f=½π√LC). This causes EMI radiationnoise to be emitted outside the switching power supply 1.

To follow is a detailed description of a printed circuit board (and amethod of patterning on it in particular) that allows effectivesuppression of the EMI radiation noise mentioned above.

Printed Circuit Board: FIGS. 6 to 9 are diagrams showing oneconfiguration example of a printed circuit board 100 that employs anovel circuit board layout devised to suppress EMI radiation noise. FIG.6 depicts a planar layout of a first wiring pattern 110 in a first layerin the printed circuit board 100. FIG. 7 depicts a planar layout of asecond wiring pattern 120 in a second layer in the printed circuit board100. FIG. 8 depicts a spatial layout of a 3D wiring pattern which is thecombination of the first and second wiring patterns 110 and 120. FIG. 9depicts exclusively the direction of a current that flows across the 3Dwiring pattern.

First, with reference to FIG. 6, the planar layout of the first wiringpattern 110 will be described. The first wiring pattern 110 is laid in afirst layer (that is, the top layer on the obverse side of the circuitboard, where components are mounted) such that, when predeterminedcomponents (in the illustrated example, a semiconductor device 10 havingthe high-side and low-side switches QH and QL integrated together, andthe bypass capacitor Cb) are mounted in a predetermined mounting region(indicated by a broken-line box in the diagram), a first current path inthe shape of an open ring that leads from a first end 111 to a secondend 112 is formed, with the first and second ends 111 and 112 locatednext to each other.

In the illustrated example, the first wiring pattern 110 is separatedinto three segments (a first segment laid between the first terminal ofthe bypass capacitor Cb and a power terminal VIN of the semiconductordevice 10, a second segment laid between the first end 111 and thesecond terminal of the bypass capacitor Cb, and a third segment laidbetween the second end 112 and a ground terminal GND of thesemiconductor device 10), and these segments are as a whole disposed inthe shape of the letter “C”.

Next, with reference to FIG. 7, the planar layout of the second wiringpattern 120 will be described. The second wiring pattern 120 is laid ina second layer (an intermediate layer inside the circuit board, or thebottom layer on the reverse side of the circuit board), which isdifferent from the first layer, such that a second current path in theshape of an open ring that leads from a third end 121 to a fourth end122 is formed, with the third and fourth ends 121 and 122 located nextto each other.

Next, with reference to FIG. 8, the spatial layout of the 3D wiringpattern which is the combination of the first and second wiring patterns110 and 120 will be described. As indicated by broken lines in thediagram, between the second end 112 of the first wiring pattern 110 andthe third end 121 of the second wiring pattern 120, a first conductivemember 131 is formed to electrically connect them together. Likewise,between the first end 111 of the first wiring pattern 110 and the fourthend 122 of the second wiring pattern 120, a second conductive member 132is formed to electrically connect them together. The first and secondconductive members 131 and 132 can each be formed using an interlayervia hole or an intermediate layer.

The first and second wiring patterns 110 and 120 are laid such that, asseen in their respective plan views, at least part of a first region(see face ABCD in FIG. 9) surrounded by the first current path overlapsat least part of a second region (see face EFGH in FIG. 9) surrounded bythe second current path. In the illustrated example in particular, thesecond wiring pattern 120 and the first wiring pattern 110 are laidparallel to each other such that the above-mentioned first and secondregions overlap completely.

Moreover, in the illustrated example, the third end 121 of the secondwiring pattern 120 is located right under (or generally right under) thesecond end 112 of the first wiring pattern 110, and the first conductivemember 131 which electrically connects them together is formed in astraight line (or generally in a straight line). Likewise, the fourthend 122 of the second wiring pattern 120 is located right under (orgenerally right under) the first end 111 of the first wiring pattern110, and the second conductive member 132 which electrically connectsthem together is formed in a straight line (or generally in a straightline).

Next, with reference to FIG. 9, the direction of the current that flowsacross the 3D wiring pattern will be described. The vertices A to H ofthe hexahedron shown in the diagram correspond to the parts indicated bythe signs A to H in FIGS. 6 to 8.

Vertex A can be considered separately for vertex A1 corresponding to thefirst end 111 and vertex A2 corresponding to the second end 112. VertexE can be considered separately for vertex E1 corresponding to the thirdend 121 and vertex E2 corresponding to the fourth end 122.

As illustrated, the current that flows across the 3D wiring pattern inthe printed circuit board 100 (that is, the differential current betweenthe high-side and low-side currents IHG and ILG in FIG. 3) flows in thedirection from vertex A (vertex A1) to vertex B to vertex C to vertex Dto vertex A (vertex A2) to vertex E (vertex E1) to vertex F to vertex Gto vertex H to vertex E (vertex E2) to vertex A (vertex A1).

That is, the first and second wiring patterns 110 and 120 are laid suchthat, as seen in their respective plan views, the direction of thecurrent that flows across the first current path (clockwise, i.e., fromvertex A (vertex A1) to vertex B to vertex C to vertex D to vertex A(vertex A2)) and the direction of the current that flows across thesecond current path (counter-clockwise, i.e., from vertex E (vertex E1)to vertex F to vertex G to vertex H to vertex E (vertex E2)) areopposite to each other.

To follow is a specific description of the effect of the above-described3D wiring pattern in cancelling magnetic fields, discussed separatelyfor each of a plurality of locations.

Magnetic Field Canceling Effect: FIG. 10 is a diagram showing themagnetic field canceling effect (with no twist) at side AE (showing sideAE as seen from a side of the first wiring pattern 110). As illustrated,the current I1 that flows across the first conductive member 131 fromthe second end A2 (112) to the third end E1 (121) and the current 12that flows across the second conductive member 132 from the fourth endE2 (122) to the first end A1 (111) flow in mutually opposite directions.

Thus, in region α (that is, elsewhere than in region β located betweenthe first and second conductive members 131 and 131), the magnetic fieldZ1 that occurs around the current I1 and the magnetic field Z2 thatoccurs around the current 12 cancel each other. This produces a magneticfield canceling effect at side AE.

In region β, the magnetic fields Z1 and Z2 act to boost each other.However, the first and second conductive members 131 and 132 are laidparallel to each other at an extremely close distance from each other,and thus region β is an extremely limited region. Accordingly, it cansafely be said that the magnetic fields Z1 and Z2 are canceled nearlycompletely.

FIG. 11 is a diagram showing the magnetic field canceling effect (with atwist) at side AE. In the illustrated example, the first and secondconductive members 131 and 132 are twisted around each other like atwisted-pair communication cable. Such a twisted structure can easily beobtained using an interlayer via hole and an intermediate layer (thatis, at least one layer provided between the first and second layers).

Owing to the above-described twisted structure, the magnetic field Z3(in the illustrated example, the magnetic field pointing from the farside to the near side of the plane of the diagram) boosted in region β1and the magnetic field Z4 (in the illustrated example, the magneticfield pointing from the near side to the far side of the plane of thediagram) boosted in region β2 cancel each other. This produces astronger magnetic field canceling effect and, ideally, makes it possibleto completely cancel the magnetic fields at side AE.

FIG. 12 is a diagram showing the magnetic field canceling effect betweenfaces ABCD and EFGH. At face ABCD, a current flows clockwise across acurrent path from vertex A to vertex B to vertex C to vertex D to vertexA. Accordingly, at face ABCD, a magnetic field Z(ABCD) that points fromtop to bottom across the plane of the diagram occurs. On the other hand,at face EFGH, a current flows counter-clockwise across a current pathfrom vertex E to vertex F to vertex G to vertex H to vertex E.Accordingly, at face EFGH, a magnetic field Z(EFGH) that points frombottom to top across the plane of the diagram occurs. As a result, themagnetic fields Z(ABCD) and Z(EFGH) cancel each other. This produces amagnetic field canceling effect between the magnetic fields Z(ABCD) andZ(EFGH).

FIG. 13 is a diagram showing the magnetic field canceling effect betweenfaces ADFE and BCGH. At face ADFE, a current that points from vertex Dto vertex A and a current that points from vertex E to vertex F flowparallel to each other, one over the other. Accordingly, at face ADFE, amagnetic field Z(ADFE) that points from the far side to the near side ofthe plane of the diagram occurs. On the other hand, at face BCGH, acurrent that points from vertex B to vertex C and a current that pointsfrom vertex G to vertex H flow parallel to each other, one over theother. Accordingly, at face BCGH, a magnetic field Z(BCGH) that pointsfrom the near side to the far side of the plane of the diagram occurs.As a result, the magnetic fields Z(ADFE) and Z(BCGH) cancel each other.This produces a magnetic field canceling effect between faces ADFE andBCGH.

FIG. 14 is a diagram showing the magnetic field canceling effect betweenfaces ABHE and DCGF. At face ABHE, a current that points from vertex Ato vertex B and a current that points from vertex H to vertex E flowparallel to each other, one over the other. Accordingly, at face ABHE, amagnetic field Z(ABHE) that points from right to left across the planeof the diagram occurs. On the other hand, at face DCGF, a current thatpoints from vertex C to vertex D and a current that points from vertex Fto vertex G flow parallel to each other, one over the other.Accordingly, at face DCGF, a magnetic field Z(DCGF) that points fromleft to right across the plane of the diagram occurs. As a result, themagnetic fields Z(ABHE) and Z(DCGF) cancel each other. This produces amagnetic field canceling effect between faces ABHE and DCGF.

As described above, owing to a 3D wiring pattern with a hexahedralstructure, it is possible to cancel all the magnetic fields that occurat six faces respectively, and it is thus possible to effectivelysuppress the EMI radiation noise from the switching power supply 1.Moreover, as opposed to the second conventional example (FIG. 17)described earlier, here, only one bypass capacitor Cb suffices. Thishelps avoid an unnecessary increase in cost.

The printed circuit board 100 employing the layout described above canbe used in switching power supplies in general, and is particularlysuitable, it can be said, for vehicle-mounted switching power supplies,which are required to pass strict tests (to comply with, for example,the CISPR25 standard).

Although the above description deals with, as one preferable embodimentthat allows one to make the most of a magnetic field canceling effect, aconfiguration where a printed circuit board 100 has a 3D wiring patternwith a hexahedral structure, this is intended merely to give an idealexample; needless to say, the 3D wiring pattern can be modified in anymanner so long as a desired magnetic field canceling effect can beobtained.

Where a more thorough approach against EMI is required, it is possibleto make, as desired, modifications such as disposing the bypasscapacitor Cb near the semiconductor device 10 and applying spectrumspreading to the driving frequency of the switching output stage (thehigh-side and low-side switches QH and QL).

Application to Split Power MOSFETs: FIG. 15 is a diagram showing a 3Dwiring pattern for configuration where high-side and low-side switchesQH and QL are each split into two parts. In the switching power supply 1shown in the diagram, inside a semiconductor device 20, a high-sideswitch QH and a low-side switch QL are each split into two parts, namelyhigh-side switches QH1 and QH2 and low-side switches QL1 and QL2respectively. Thus, in this configuration, the high-side and low-sideswitches QH and QL each have a reduced AL impedance.

In the switching power supply 1 of this configuration example, asindicated in the balloon in the diagram, by providing one set of thepreviously described 3D wiring pattern (a first wiring pattern 110, asecond wiring pattern 120, a first conductive member 131, and a secondconductive member 132) for each of the input loops disposedsymmetrically left to right (that is, by providing a total of two suchsets), it is possible to obtain a magnetic field canceling effectsimilar to that described previously.

With the switching power supply 1 of this configuration example, amagnetic field canceling effect is obtained for each of the left andright 3D wiring patterns. Thus, even where the left and right inputloops have different sizes, or are located far away from each other, nonotable effect appears in the individual magnetic field cancelingeffects. That is, the switching power supply 1 of this configurationexample, as opposed to the second conventional example (FIG. 17)described earlier, is not intended to make deliberate use of thecancelling effect between the magnetic fields occurring in the left andright input loops respectively.

Further Modifications: The various technical features disclosed in thepresent specification can be implemented in any other ways than as inthe embodiment described above, and allow of any modifications withoutdeparture from the spirit of the technical ingenuity of the invention.

For example, the type of operation of the switching power supply is notlimited to a step-down type; it may instead be a step-up type, astep-up/down type, or an inverting type. The type of rectification ofthe switching power supply may also be of any type; instead ofsynchronous rectification, diode rectification may be employed.

Also the type of output feedback control of the switching power supplyis not subject to any restriction; it is possible to employ any outputfeedback control (such as voltage mode control, current mode control,bottom-detecting on-time control, peak-detecting off-time control, orhysteresis control).

Thus, the embodiment described above should be considered to be in everyaspect illustrative and not restrictive, and the technical scope of thepresent invention should be understood to encompass any modificationswithin the sense and scope equivalent to those of the appended claims.

Industrial Applicability: Switching power supplies according to what isdisclosed herein can be used suitably as, for example, vehicle-mountedpower supplying means that are required to comply with a strict noisestandard.

What is claimed is:
 1. A printed circuit board comprising: a firstwiring pattern; and a second wiring pattern, wherein a first ring-shapedcurrent path is formed across the first wiring pattern, a secondring-shaped current path is formed across the second wiring pattern, adirection of a current flowing across the first ring-shaped current pathand a direction of a current flowing across the second ring-shapedcurrent path are opposite to each other, the first and secondring-shaped current paths are located above each other in a thicknessdirection of the printed circuit board, at least part of a first regionsurrounded by the first ring-shaped current path and at least part of asecond region surrounded by the second ring-shaped current path overlapeach other as seen in a plan view of the printed circuit board.
 2. Theprinted circuit board according to claim 1, further comprising: a firstconductive member connected to the first ring-shaped current path, thefirst conductive member letting a current flow in a first directionpointing from the first ring-shaped current path to the secondring-shaped current path.
 3. The printed circuit board according toclaim 2, further comprising: a second conductive member connected to thesecond ring-shaped current path, the second conductive member letting acurrent flow in a second direction pointing from the second ring-shapedcurrent path to the first ring-shaped current path.
 4. The printedcircuit board according to claim 3, wherein the first and secondconductive members are located next to each other.
 5. The printedcircuit board according to claim 1, wherein a capacitor is disposed inat least one of the first and second ring-shaped current paths.
 6. Theprinted circuit board according to claim 1, wherein a switching deviceis disposed in at least one of the first and second ring-shaped currentpaths.
 7. The printed circuit board according to claim 2, wherein thefirst conductive member is connected also to the second ring-shapedcurrent path.
 8. The printed circuit board according to claim 3, whereinthe second conductive member is connected also to the first ring-shapedcurrent path.
 9. The printed circuit board according to claim 1, whereinthe first and second ring-shaped current paths are both in a shape of anopen ring.
 10. The printed circuit board according to claim 3, whereinthe first and second conductive members are twisted around each other.11. The printed circuit board according to claim 3, comprising aplurality of sets each comprising the first wiring pattern, the secondwiring pattern, the first conductive member, and the second conductivemember.
 12. A switching power supply comprising: the printed circuitboard according to claim 1; and a switching output stage and a bypasscapacitor mounted on the printed circuit board to form, along with thefirst wiring pattern, the first ring-shaped current path.
 13. Theswitching power supply according to claim 12, wherein the switchingoutput stage is integrated into a semiconductor device.
 14. Theswitching power supply according to claim 13, wherein the bypasscapacitor is disposed near the semiconductor device.
 15. The switchingpower supply according to claim 12, wherein spectrum spreading isapplied to a driving frequency of the switching output stage.
 16. Anelectronic appliance comprising: the switching power supply according toclaim 12; and a load operating by being supplied with electric powerfrom the switching power supply.